Toll and Toll-like receptors (TLRs) have a central role in innate immune responses in animals ranging from insects to humans. Whereas we know a great deal about the pathogen recognition that initiates Toll signaling and the signaling mechanism itself, we know considerably less about the effectors that mediate innate immune defenses. To address this need, we carried out a cross- species comparative analysis of gene expression to identify conserved features of the Toll- regulated immune repertoire in Drosophila. Focusing our attention on a novel family of immune effectors identified in this manner, we used a state-of-the-art genomic engineering approach to eliminate function in ten of twelve family members. Remarkably, inactivating these ten genes decreased survival upon microbial infection to the same extent and with the same specificity as a complete block in Toll signaling. These findings establish the essential immune function of what we have named the Bom gene family and provide the basis for an original and innovative analysis of effector peptide function. By working in Drosophila, we can readily generate mutations that disrupt pathway activity, monitor and manipulate gene activity, and map out networks of gene function via molecular, biochemical, and bioinformatic approaches. Using a combination of transgenesis and mutagenesis, we will match Bom-mediated defenses to particular pathogens and delineate the structure-function relationships among the Bom peptides. We will then apply these findings to the design and interpretation of in vitro studies of peptide function, focusing on antimicrobial activity. Next, we will explore Bom gene sufficiency in vivo and delineate the extent to which the function of Bom and previously described antimicrobial peptides intersect or overlap. Finally, we will begin mapping out the broader network of Toll mediated defenses by genetic and phenotypic analysis of clusters of other novel effector genes.